Today's fiber optic based networks use transceivers as the interface between electronics and optical signals that propagate on the optical fiber and at other points in a network in which information is converted between electronic form and optical form.
The optical communication device including photonic and/or optoelectronic devices and components used to transmit, code, receive, decode optical data for transmission on an optical fiber, are interfaced to a variety of electronic circuits used to control these elements as well as to interface data in electronic form on the transmit and receive side, code and decode electronic data, perform other functions like clock recovery and error correction and realize functions required to control the environment of these circuits including temperature.
The challenges that exist today for tunable lasers, modules, and/or subassemblies, based on tunable lasers, are in part in the complexity and time involved in calibrating output characteristics of an output, in particular, optical frequency (or wavelength) in response to an applied control signal (for example voltage, current, temperature or any combination thereof). Depending on a laser design and tuning physics, a control method and control signals can vary widely. There are several classes of tunable lasers, including those designs that can tune to any desired wavelength (within some specified accuracy) over a wide range of tuning, for example, a C-band or O-band for communications systems.
The cost of manufacturing tunable lasers and making their wavelength calibration fast and robust is critical in replacing fixed wavelength lasers with tunable lasers as well as opening new applications and markets. A need to move to tunable lasers in systems where many wavelengths or channels are transmitted on a single fiber has become an economical and practical necessity because carrying an inventory of all fixed wavelength lasers, the infrastructure needed to support specifying and carrying this inventory as well as the cost of having a downtime for a channel due to non-availability and mistake made in shipping the wrong wavelength lasers to the field become significant factors for today's high capacity networks where each fiber can carry, 40, 80, 96, 128 or more wavelengths.
Additionally, the cost of new modules that are configured to transmit data at 100 Giga bits per second (Gbps), 200 Gbps, 400 Gbs and faster, makes it necessary to use a tunable laser such that one module type can be used to access any channel on the fiber channel. For these new high capacity systems it has become prohibitively expensive to deploy these interfaces with fixed wavelength lasers and the industry is moving in a direction of tunable lasers for single wavelength high bit rate modules and interfaces.
One widely used class of tunable laser that can be monolithically integrated onto a photonic integrated circuit (PIC), such as that described in U.S. Patent Provisional Application Ser. No. 61/748,415, which is incorporated herein by reference in its entirety, belongs to the quasi-continuous tuning class of laser, which is defined as a laser capable of reaching any desired wavelength with a control system that needs to control multiple sections of the laser with set of control signals that are mapped between control signals and wavelengths. Often, the control methods used to tune quasi-continuous lasers can be complex, with complex relationships between control signals and output wavelength, and thus techniques like lookup tables are utilized where the full tuning maps for all control signals and desired wavelengths are stored during a calibration time. The quasi-continuous laser type is differentiated from the contiguous laser type in that the later can be tuned continuously with an adjustment of a single knob or single control signal that can sweep through all output frequencies or wavelengths, and the control and often time calibration are simplified over the quasi-contiguous laser.
A primary issue with tunable lasers, and in particular quasi-continuous lasers like that described in U.S. Patent Provisional Application Ser. No. 61/748,415, however, is the time it takes to fully calibrate the tunable laser in terms of control signals and output wavelengths. This calibration must be fast, must not become a bottleneck in the manufacturing process or will significantly drive up the laser cost and manufacturing throughput, but will also affect the robustness of the calibration, a degree of process automation and robustness to design tolerances, surrounding control circuits and optics, process variations and laser operating and aging variations.
As tunable lasers and other components are more tightly integrated (like optical data modulators and wavelength lockers for example), fast calibration times and associated methods and apparatus will become critical to wide-scale deployment of tunable laser in terms of cost, integration of the laser into other subassemblies and systems. Additionally, the speed of laser wavelength calibration can affect where the calibration can occur, and fast techniques leave open the possibility to calibrate not only at the manufacturer of the laser, but at equipment manufacturers that build the laser based component into a system, and even recalibration of the laser in the system out in the field.
An aspect of today's technology is that for calibration of a tunable laser, linear monolithic code running on microprocessors and/or state machines is used for a calibration routine, and the results of the calibration may then be stored in a lookup table used to map control signals to a desired output wavelength. However, there are several drawbacks with this approach, one is the efficiency (lack of speed) in using a personal computer (PC) or microprocessor approach to execute the calibration routine and control and interface to all of the tooling and equipment as well as the tunable laser or the device to be calibrated. Another drawback is the uniformity of manufacturing, where in the end the calibration data may be stored in an FPGA, and the ability to run calibration routines during manufacturing and once the tunable laser based product is utilized in a communications systems requires a separate microprocessor for wavelength calibration. Uniformity of code, modulatory of code, uniformity of hardware in the manufacturing and calibration line as well as in the deployment line, leads to economic and other efficiencies over existing technologies.
The primary drawbacks to existing wavelength calibration methods and apparatus are related in part to a laser design and techniques enabling the calibration process as well as automation and software control of the calibration. These calibration drawbacks limit the market for tunable laser and optical subassemblies and communications modules employing such lasers, by increasing the cost, time and complexity to manufacture as well as limitations to re-calibration while being installed in a product or deployed in the field.
Therefore there is a need for new high speed tunable laser calibration techniques, algorithms and implementations to lower the cost and time of manufacturing and providing increased automation of integrating such tunable lasers into systems, communications systems and networks.